Patients are having to wait up to 550 days for routine NHS scans, while many of the machines are lying unused for hours each week.

The postcode lottery operating over hospitals' diagnostic services is exposed today in a nationwide survey of hospitals. It shows enormous differences in waiting times between parts of England, despite the government's attempts to iron out the disparities.

There are five other hospitals, including the Royal Bolton Hospital, the North Middlesex University Hospital and Derby City General Hospital, where the waits are more than a year long.

MRI scans are given to diagnose a range of illnesses, from back and knee injuries to brain problems or epilepsy. They can help identify conditions such as cancer, multiple sclerosis and arthritis.

However, the demand for the scans is now exceeding the capacity within the NHS, also exacerbated because of a shortage of radiographers to operate the machines.

The problem is compounded by the fact that many of the scanners remain unused for hours because the revenue funding for the scans is limited.

At Walsall, the MRI scanner is provided by the private company Lister InHealth, which also provides and runs scanners for other hospitals.

A spokeswoman for the hospital said that its wait for a routine scan was particularly long because it had been prioritising emergency and routine work. However, it is understood that Lister has been unable to operate the scanner as often as it wanted to because of a lack of money to screen extra patients.

A spokesman for Lister refused to give details on how many patients it was funded to treat, but said: 'We can only do the work we are contracted to do. 'There has been a lot of emergency work going through, but it is true to say that we do have some spare capacity.' However, the Health secretary, John Reid, pointed to the fact that the average wait for a routine scan in the UK had fallen to 132 days, from 154 in 2001-2002.

The picture is uneven, however. Since the first survey by the health information group Dr Foster, the MRI waits have worsened in 58 English hospitals and improved in 57 others.

The £1 million machines, often bought by charitable trusts attached to the hospitals, are in short supply. Earlier this year, the government invited bids from private firms to supply and run them.

Dr Foster's research director, Roger Taylor, told the Health Service Journal magazine: 'These are expensive machines and it seems difficult to justify such long waits, even for routine scans, when they are spending a lot of time not being used.'